[SOLVED] Python type hint for undefined values to distinguish from None

Python type hint for undefined values to distinguish from None

I want to create a function signature where an undefined value and None are distinct from each other.

So e.g.:

undefined = object()

def update(id: str, title: str = undefined, description: str):
    data = {
        "title": title,
        "description": description,
    }

    # remove undefined key-value pairs from data
    for key in data.keys():
        if data[key] is undefined:
            del data[key]
    
    # send data to the server for further processing

Obviously this way I get conflicting types and the following error: Incompatible default for argument "description" (default has type "object", argument has type "str")

From the above example it can be seen that passing None is completely valid as it will be replaced as the null value of JSON while if undefined is passed then the value won't be considered in the data.

I've tried to define undefined types like:

UndefinedStr = typing.Union[str, undefined]

But I got the following error: Variable "undefined" is not valid as a type

I guess that's because undefined is an instance. So as a workaround I could do:

class Undefined:
    pass

undefined = Undefined()

UndefinedStr = Union[str, Undefined]

This solution works, but to me feels a bit too broad.

So my question what is the best way to type hint this undefined use-case?

UPDATE:

Please don't focus on the fact that in the example the type of the title argument is str, the main intention behind this question is to find a universal type hint for a sentinel object that could be used in union with any other type.

In the meantime I've seen some progress on this specific issue within PEP-0661, so seemingly this has become a broader topic to discuss.

Solution

So far the best proposal is provided by PEP-661. Let's check that out and monitor it for a possible implementation.